Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)Impedance Mismatchmarc@petit-huguenin.orgCisco Systems707 Tasman DrMilpitasCA95035USAsnandaku@cisco.comEricssonJorvas02420Finlandari.keranen@ericsson.comRAI
MMUSIC
This document describes Session Description Protocol (SDP) Offer/Answer procedures
for carrying out Interactive Connectivity Establishment (ICE) between the agents.
This document describes how Interactive Connectivity Establishment (ICE) is used
with Session Description Protocol (SDP) offer/answer . The ICE specification
describes procedures that are common to all usages of ICE and this document
gives the additional details needed to use ICE with SDP offer/answer.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119.
Readers should be familiar with the terminology defined in ,
in and the following:
The default destination for a component of a data stream is the transport
address that would be used by an agent that is not ICE aware. A default
candidate for a component is one whose transport address matches the default
destination for that component. For the RTP component, the default IP address
is in the "c=" line of the SDP, and the port is in the "m=" line. For the
RTCP component, the address and port are indicated using the "a=rtcp"
attribute defined in , if present; otherwise, the RTCP component
address is same as the address of the RTP component, and its port is one
greater than the port of the RTP component.
defines ICE candidate exchange as the process for ICE
agents (Initiator and Responder) to exchange their candidate information
required for ICE processing at the agents. For the purposes of this
specification, the candidate exchange process corresponds to the
Offer/Answer protocol and the terminologies offerer and answerer correspond
to the initiator and responder terminologies from respectively.
Once the initiating agent has gathered, pruned and prioritized its
set of candidates , the candidate exchange with the peer agent begins.
provides detailed rules for constructing various SDP
attributes defined in this specification.
Each data stream is represented by an SDP media description ("m=" section).
With in a "m=" section, each candidate (including the default candidate) associated
with the data stream is represented by an SDP candidate attribute.
Prior to nomination, the "c=" line associated with an "m=" section contains
the IP address of the default candidate, while the "m=" line contains the port
and transport of the default candidate for that "m=" section.
After nomination, the "c=" line for a given "m=" section contains the IP address
of the nominated candidate (the local candidate of the nominated candidate pair)
and the "m=" line contains the port and transport corresponding to the
nominated candidate for that "m=" section.
The ICE username is represented by an SDP ice-ufrag attribute and the ICE
password is represented by an SDP ice-pwd attribute.
An ICE lite implementation MUST include an SDP ice-lite attribute.
A full implementation MUST NOT include that attribute.
An agent uses the SDP ice-options attribute to indicate support of ICE
extensions.
An agent compliant to this specification MUST include an SDP ice-options
attribute with an "ice2" attribute value. If an agent receives an SDP offer
or answer with ICE attributes but without the "ice2" ice-options attribute value,
the agent assumes that the peer is compliant to .
If an "m=" section is marked as inactive , or has a bandwidth
value of zero , the agent MUST still include ICE related SDP
attributes.
If the port value associated with an "m=" section is set to zero (implying a
disabled stream) as defined in section 8.2 of , the agent SHOULD NOT
include ICE related SDP candidate attributes in that "m=" section, unless an
SDP extension specifying otherwise is used.
If an agent utilizes both RTP and RTCP, the agent MUST include SDP candidate
attributes for both the RTP and RTCP components in the "m=" section.
If an agent uses separate ports for RTP and RTCP, the agent MUST include an
SDP rtcp attribute in the "m=" section, as described in . In the cases
where the port number for the RTCP is one higher than the RTP port and RTCP
component address is same as the address of the RTP component, the SDP rtcp
attribute MAY be omitted.
If the agent does not utilize RTCP, it indicates that by including b=RS:0
and b=RR:0 SDP attributes, as described in .
The offerer acts as the Initiating agent. The answerer acts as the
Responding agent. The ICE roles (controlling and controlled) are determined
using the procedures in .
Once an agent has provided its local candidates to its peer in an SDP
offer or answer, the agent MUST be prepared to receive STUN connectivity
check Binding requests on those candidates.
The agents will proceed with the ICE procedures defined in and
this specification if, for each data stream in the SDP it received, the
default destination for each component of that data stream appears in
a candidate attribute. For example, in the case of RTP, the IP address
and port in the "c=" and "m=" lines, respectively, appear in a candidate
attribute and the value in the rtcp attribute appears in a candidate
attribute.
If this condition is not met, the agents MUST process the SDP based on
normal procedures, without using any of the ICE mechanisms
described in the remainder of this specification with the few
exceptions noted below:
The presence of certain application layer gateways MAY modify
the transport address information as described in .
The behavior of the responding agent in such a situation is
implementation defined. Informally, the responding agent MAY
consider the mismatched transport address information as a
plausible new candidate learnt from the peer and continue its
ICE processing with that transport address included.
Alternatively, the responding agent MAY include an "a=ice-mismatch"
attribute in its answer and MAY also omit a=candidate attributes for
such data streams.
The transport address from the peer for the default destination
correspond to IP address values "0.0.0.0"/"::" and port value of "9".
This MUST not be considered as a ICE failure by the peer agent and
the ICE processing MUST continue as usual.
Also to note, this specification provides no guidance on how an
controlling/initiator agent should proceed in scenarios where the
the SDP answer includes "a=ice-mismatch" from the peer.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
When an offerer generates the initial offer, in each "m=" section it MUST
include SDP candidate attributes for each available candidate associated
with the "m=" section. In addition, the offerer MUST include an SDP ice-ufrag
and an SDP ice-pwd attribute in the offer.
Within the scope of this document, "Initial Offer" refers to the first
SDP offer that is sent in order to negotiate usage of ICE. It might, or
might not, be the initial SDP offer of the SDP session.
The procedures in this document only consider "m=" sections associated
with data streams where ICE is used.
When an answerer receives an initial offer that indicates that the offerer
supports ICE, and if the answerer accepts the offer and the usage of ICE,
in each "m=" section within the answer, it MUST include SDP candidate
attributes for each available candidate associated with the "m=" section.
In addition, the answerer MUST include an SDP ice-ufrag and an SDP ice-pwd
attribute in the answer.
Once the answerer has sent the answer, it can start performing connectivity
checks towards the peer candidates that were provided in the offer.
If the offer does not indicate support of ICE, the answerer
MUST NOT accept the usage of ICE. If the answerer still accepts the offer,
the answerer MUST NOT include any ICE related SDP attributes in the answer.
Instead the answerer will generate the answer according to normal
offer/answer procedures .
If the answerer detects a possibility of the ICE mismatch, procedures
described in () are followed.
When an offerer receives an initial answer that indicates that the answerer
supports ICE, it can start performing connectivity checks towards the peer
candidates that were provided in the answer.
If the answer does not indicate that the answerer supports ICE, or if the
offerer detects an ICE mismatch in the answer, the offerer MUST terminate
the usage of ICE. The subsequent actions taken by the offerer are
implementation dependent and are out of the scope of this specification.
Once the state of each check list is Completed, and if the agent is
the controlling agent, it nominates a candidate pair and
checks for each data stream whether the nominated pair matches the default
candidate pair. If there are one or more data streams with a match, and
the peer did not indicate support for the 'ice2' ice-option,
the controlling agent MUST generate a subsequent offer
(), in which the IP address, port and
transport in the "c=" and "m=" lines associated
with each data stream match the corresponding local information of the
nominated pair for that data stream.
However, If the support for 'ice2' ice-option is in use, the nominated
candidate is noted and sent in the subsequent offer/answer exchange as
the default candidate and no updated offer is needed to fix the default
candidate.
Also as described in , once the controlling agent has nominated
a candidate pair for a data stream, the agent MUST NOT nominate another pair
for that data stream during the lifetime of the ICE session (i.e. until
ICE is restarted).
Either agent MAY generate a subsequent offer at any time allowed by
. This section defines rules for construction of subsequent
offers and answers.
Should a subsequent offer fail, ICE processing continues as if the
subsequent offer had never been made.
An agent MAY restart ICE processing for an existing data stream .
The rules governing the ICE restart imply that setting the IP address in the
"c=" line to 0.0.0.0 (for IPv4)/ :: (for IPv6) will cause an ICE restart.
Consequently, ICE implementations MUST NOT utilize this mechanism for call hold,
and instead MUST use a=inactive and a=sendonly as described in .
To restart ICE, an agent MUST change both the ice-pwd and the ice-ufrag for
the data stream in an offer. However, it is permissible to use a session-level
attribute in one offer, but to provide the same ice-pwd or ice-ufrag as a
media-level attribute in a subsequent offer. This MUST NOT be considered
as ICE restart.
An agent sets the rest of the ice related fields in the SDP for this data stream as it
would in an initial offer of this data stream (see ).
Consequently, the set of candidates MAY include some, none, or all of the
previous candidates for that data stream and MAY include a totally new set of
candidates.
If an agent removes a data stream by setting its port to zero, it MUST NOT
include any candidate attributes for that data stream and SHOULD NOT include
any other ICE-related attributes defined in for that data stream.
If an agent wishes to add a new data stream, it sets the fields in the SDP for
this data stream as if this was an initial offer for that data stream
(see ). This will cause ICE processing to begin for this data stream.
This section describes additional procedures for full implementations,
covering existing data streams.
When an offerer sends a subsequent offer; in each "m=" section for which a
candidate pair has not yet been nominated, the offer MUST include the
same set of ICE-related information that the offerer included in the
previous offer or answer. The agent MAY include additional candidates
it did not offer previously, but which it has gathered since the last
offer/ answer exchange, including peer reflexive candidates.
The agent MAY change the default destination for media. As with initial
offers, there MUST be a set of candidate attributes in the offer matching
this default destination.
Once a candidate pair has been nominated for a data stream, the IP address,
port and transport in each "c=" and "m=" line associated with that data
stream MUST match the data associated with the nominated pair for that
data stream. In addition, the offerer only includes SDP candidates
representing the local candidates of the nominated candidate pair. The
offerer MUST NOT include any other SDP candidate attributes in the
subsequent offer.
In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each data stream whose check list
is in the completed state. The attribute contains the remote candidates
corresponding to the nominated pair in the valid list for each
component of that data stream. It is needed to avoid a race condition
whereby the controlling agent chooses its pairs, but the updated offer
beats the connectivity checks to the controlled agent, which doesn’t
even know these pairs are valid, let alone selected. See Appendix B
for elaboration on this race condition.
If the ICE state is running, a lite implementation MUST include all of
its candidates for each component of each data stream in a=candidate
attribute in any subsequent offer. The candidates are formed identical
to the procedures for initial offers.
A lite implementation MUST NOT add additional host candidates in a
subsequent offer. If an agent needs to offer additional candidates,
it MUST restart ICE. Similarly, the username fragments or passwords
MUST remain the same as used previously. If an agent needs to change
one of these, it MUST restart ICE for that media stream.
If ICE has completed for a data stream and if the agent is controlled,
the default destination for that data stream MUST be set to the
remote candidate of the candidate pair for that component in the valid list.
For a lite implementation, there is always just a single candidate pair in
the valid list for each component of a data stream. Additionally, the agent
MUST include a candidate attribute for each default destination.
If ICE state is completed and if the agent is controlling (which only
happens when both agents are lite), the agent MUST include the
a=remote-candidates attribute for each data stream. The attribute
contains the remote candidates from the candidate pairs in the
valid list (one pair for each component of each data stream).
If ICE is Completed for a data stream, and the offer for that data
stream lacked the a=remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer,
except that the answerer MUST NOT include the a=remote-candidates
attribute in the answer.
A controlled agent will receive an offer with the a=remote-candidates
attribute for a data stream when its peer has concluded ICE processing
for that data stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding Response that tells the answerer the
candidate that will be selected by ICE. See Appendix B for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the data stream by:
Setting the remote candidate equal to the offerer’s default
destination for that component (i.e. the contents of the "m=" and
"c=" lines for RTP, and the a=rtcp attribute for RTCP)
Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs is present
in the valid list. If a particular pair is not in the valid list,
the check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the check list whose remote
candidates equal the remote candidate in the losing pair:
If none of the pairs are In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this data stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
If at least one of the pairs is In-Progress, the agent SHOULD wait
for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
If the offerer in a subsequent offer requested an ICE restart
for a data stream, and if the answerer accepts the offer, the
answerer follows the procedures for generating an initial answer.
For a given data stream, the answerer MAY include the same
candidates that were used in the previous ICE session, but
it MUST change the SDP ice-pwd and ice-ufrag attribute
values.
If the received offer contains the remote-candidates attribute for a
data stream, the agent forms a candidate pair for each component of the
data stream by:
Setting the remote candidate equal to the offerer’s default destination
for that component (i.e., the contents of the "m=" and "c=" lines for RTP,
and the a=rtcp attribute for RTCP).
Setting the local candidate equal to the transport address for that same
component in the a=remote-candidates attribute in the offer.
The state of ICE processing for that data stream is set to Completed.
Furthermore, if the agent believed it was controlling, but the offer contained
the a=remote-candidates attribute, both agents believe they are controlling.
In this case, both would have sent updated offers around the same time.
However, the signaling protocol carrying the offer/answer exchanges
will have resolved this glare condition, so that one agent is always
the 'winner' by having its offer received before its peer has sent
an offer. The winner takes the role of controlling, so that the
loser (the answerer under consideration in this section) MUST
change its role to controlled.
Consequently, if the agent was going to send an updated offer since,
based on the rules in section 8.2 of , it was controlling,
it no longer needs to.
Besides the potential role change, change in the Valid list, and
state changes, the construction of the answer is performed identically
to the construction of an offer.
There may be certain situations where the offerer receives
an SDP answer that lacks ICE candidates although the initial answer
did. One example of such an "unexpected" answer might be
happen when an ICE-unaware B2BUA introduces a media server during
call hold using 3rd party call-control procedures. Omitting
further details how this is done, this could result in an answer being
received at the holding UA that was constructed by the B2BUA. With
the B2BUA being ICE-unaware, that answer would not include ICE
candidates.
Receiving an answer without ICE attributes in this situation might be
unexpected, but would not necessarily impair the user experience.
When the offerer receives an answer indicating support for ICE, the
offer performs on of the following actions:
If the offer was a restart, the agent MUST perform ICE restart
procedures as specified in
If the offer/answer exchange removed a data stream, or an
answer rejected an offered data stream, an agent MUST flush the
Valid list for that data stream. It MUST also terminate any
STUN transactions in progress for that data stream.
If the offer/answer exchange added a new data stream, the agent
MUST create a new check list for it (and an empty Valid list to
start of course) which in turn triggers the candidate
processing procedures .
If ICE state is running for a given data stream, the agent
recomputes the check list. If a pair on the new check list was
also on the previous check list, and its state is not Frozen,
its state is copied over. Otherwise, its state
is set to Frozen. If none of the check lists are active (meaning
that the pairs in each check list are Frozen), appropriate
procedures in are performed to move candidate(s)
to the Waiting state to further continue ICE processing.
If ICE state is completed and the SDP answer conforms to
, the agent MUST reman in the ICE
completed state.
However, if the ICE support is no longer indicated in the SDP answer,
the agent MUST fall-back to procedures and SHOULD NOT
drop the dialog because of the missing ICE support or unexpected answer.
Once the agent sends a new offer later on, it MUST perform an ICE restart.
The agent MUST remember the nominated pair in the Valid list for each
component of the data stream, called the previous selected pair prior
to the restart. The agent will continue to send media using this pair,
as described in section 12 of . Once these destinations are
noted, the agent MUST flush the valid and check lists, and then recompute
the check list and its states, thus triggering the candidate processing
procedures
If ICE is restarting for a data stream, the agent MUST start a new
Valid list for that data stream. It MUST remember the nominated pair in the
previous Valid list for each component of the data stream, called
the previous selected pairs, and continue to send media there as
described in section 12 of . The state of ICE processing
for each data stream MUST change to Running, and the state of
ICE processing MUST change to Running
This specification defines eight new SDP attributes — the "candidate",
"remote-candidates", "ice-lite", "ice-mismatch", "ice-ufrag", "ice-pwd", "ice-pacing",
and "ice-options" attributes.
This section also provides non-normative examples of the attributes defined.
The syntax for the attributes follow Augmented BNF as defined in .
The candidate attribute is a media-level attribute only.
It contains a transport address for a candidate that can be used for connectivity checks.
This grammar encodes the primary information about a candidate: its IP address, port and
transport protocol, and its properties: the foundation, component ID, priority, type, and
related transport address:
is taken from RFC 4566 .
It is the IP address of the candidate. When parsing this field, an agent can differentiate
an IPv4 address and an IPv6 address by presence of a colon in its value — the presence of
a colon indicates IPv6. An agent MUST ignore candidate lines that include candidates with
IP address versions that are not supported or recognized.
is also taken from RFC 4566 .
It is the port of the candidate.
indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is provided to allow for
future transport protocols to be used with ICE by extending the sub-registry
"ICE Transport Protocols" under "Interactive Connectivity Establishment (ICE)" registry.
is composed of 1 to 32 <ice-char>s.
It is an identifier that is equivalent for two candidates that are of the same type,
share the same base, and come from the same STUN server.
The foundation is used to optimize ICE performance in the Frozen algorithm as
described in
is a positive integer between 1 and 256 (inclusive) that
identifies the specific component of the dta stream for which this is a candidate.
It MUST start at 1 and MUST increment by 1 for each component of a particular candidate.
For data streams based on RTP, candidates for the actual RTP media MUST have a component
ID of 1, and candidates for RTCP MUST have a component ID of 2.
See section 13 in for additional discussion on extending ICE to new data streams.
is a positive integer between 1 and (2**31 - 1) inclusive. The procedures
for computing candidate’s priority is described in section 5.1.2 of .
encodes the type of candidate.
This specification defines the values "host", "srflx", "prflx", and "relay" for host,
server reflexive, peer reflexive, and relayed candidates, respectively.
Specifications for new candidate types MUST define how, if at all, various steps in the ICE
processing differ from the ones defined by this specification.
convey transport addresses related to the candidate,
useful for diagnostics and other purposes.
<rel-addr> and <rel-port> MUST be present for server reflexive, peer reflexive,
and relayed candidates. If a candidate is server or peer reflexive, <rel-addr> and
<rel-port> are equal to the base for that server or peer reflexive candidate. If the
candidate is relayed, <rel-addr> and <rel-port> are equal to the mapped address in the
Allocate response that provided the client with that relayed candidate (see
Appendix B.3 of for a discussion of its purpose).
If the candidate is a host candidate, <rel-addr> and <rel-port> MUST be omitted.
In some cases, e.g., for privacy reasons, an agent may not want to reveal the related
address and port. In this case the address MUST be set to "0.0.0.0" (for IPv4 candidates)
or "::" (for IPv6 candidates) and the port to zero.
The candidate attribute can itself be extended. The grammar allows for new name/value pairs
to be added at the end of the attribute. Such extensions MUST be made through IETF Review or
IESG Approval and the assignments MUST contain the specific extension and a
reference to the document defining the usage of the extension
An implementation MUST ignore any name/value pairs it doesn’t understand.
The syntax of the "remote-candidates" attribute is defined using Augmented BNF
as defined in .
The remote-candidates attribute is a media-level attribute only.
The attribute contains a connection-address and port for each component. The ordering
of components is irrelevant. However, a value MUST be present for each component of a
data stream. This attribute MUST be included in an offer by a controlling agent for
a data stream that is Completed, and MUST NOT be included in any other case.
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of which are flags, is:
"ice-lite" is a session-level attribute only, and indicates that an agent is a
lite implementation. "ice-mismatch" is a media-level attribute only, and when present
in an answer, indicates that the offer arrived with a default destination for a media
component that didn’t have a corresponding candidate attribute.
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment and password used by ICE for message integrity.
Their syntax is:
The "ice-pwd" and "ice-ufrag" attributes can appear at either the session-level
or media-level. When present in both, the value in the media-level takes precedence.
Thus, the value at the session-level is effectively a default that applies to all
data streams, unless overridden by a media-level value. Whether present at the session
or media-level, there MUST be an ice-pwd and ice-ufrag attribute for each data stream.
If two data streams have identical ice-ufrag’s, they MUST have identical ice-pwd’s.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the beginning of
a session (the same applies when ICE is restarting for an agent).
The ice-ufrag attribute MUST contain at least 24 bits of randomness, and the ice-pwd
attribute MUST contain at least 128 bits of randomness. This means that the ice-ufrag
attribute will be at least 4 characters long, and the ice-pwd at least 22 characters long,
since the grammar for these attributes allows for 6 bits of information per character.
The attributes MAY be longer than 4 and 22 characters, respectively, of course, up to
256 characters. The upper limit allows for buffer sizing in implementations.
Its large upper limit allows for increased amounts of randomness to be added over time.
For compatibility with the 512 character limitation for the STUN username attribute value
and for bandwidth conservation considerations, the ice-ufrag attribute MUST NOT be longer
than 32 characters when sending, but an implementation MUST accept up to 256 characters
when receiving.
The "ice-pacing" is a session level attribute that indicates the desired connectivity
check pacing, in milliseconds, for this agent (see section 14 of ).
The syntax is:
Following the procedures defined in , a default value of 50ms
is used for an agent when ice-pacing attribute is omitted in the offer or
the answer.
The same rule applies for ice-pacing attribute values lower than
50ms. This mandates that, if an agent includes the ice-pacing attribute,
its value MUST be greater than 50ms or else a value of 50ms
is considered by default for that agent.
Also the larger of the ice-pacing attribute values between the offer and
the answer (determined either by the one provided in the ice-pacing attribute
or by picking the default value) MUST be considered for a given ICE session.
The "ice-options" attribute is a session- and media-level attribute.
It contains a series of tokens that identify the options supported by the agent.
Its grammar is:
The existence of an ice-option in an offer indicates that a certain extension
is supported by the agent and is willing to use it, if the peer agent also includes
the same extension in the answer. There might be further extension specific
negotiation needed between the agents that determine how the extensions gets used
in a given session. The details of the negotiation procedures, if present, MUST be
defined by the specification defining the extension (see ).
>:
a=ice-options:rtp+ecn
]]>
All the ICE agents MUST follow the procedures defined in section 11 of
for sending keepalives. The keepalives MUST be sent regardless of whether the
data stream is currently inactive, sendonly, recvonly, or sendrecv, and regardless
of the presence or value of the bandwidth attribute. An agent can determine that its
peer supports ICE by the presence of a=candidate attributes for each media session.
Note that ICE is not intended for NAT traversal for SIP, which is assumed to be
provided via another mechanism .
When ICE is used with SIP, forking may result in a single offer generating a
multiplicity of answers. In that case, ICE proceeds completely in parallel and
independently for each answer, treating the combination of its offer and
each answer as an independent offer/answer exchange, with its own set of local
candidates, pairs, check lists, states, and so on.
Once ICE processing has reached the Completed state for all peers for media
streams using those candidates, the agent SHOULD wait an additional three seconds,
and then it MAY cease responding to checks or generating triggered checks on that
candidate. It MAY free the candidate at that time.
Freeing of server reflexive candidates is never explicit; it happens by lack of
a keepalive. The three-second delay handles cases when aggressive nomination
is used, and the selected pairs can quickly change after ICE has completed.
ICE requires a series of STUN-based connectivity checks to take place between
endpoints. These checks start from the answerer on generation of its answer,
and start from the offerer when it receives the answer.
These checks can take time to complete, and as such, the selection of
messages to use with offers and answers can affect perceived user latency.
Two latency figures are of particular interest. These are the post-pickup delay
and the post-dial delay. The post-pickup delay refers to the time between when
a user "answers the phone" and when any speech they utter can be delivered to
the caller. The post-dial delay refers to the time between when a user enters
the destination address for the user and ringback begins as a consequence of
having successfully started alerting the called user agent.
Two cases can be considered — one where the offer is present in the initial
INVITE and one where it is in a response.
To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering
candidates prior to actually sending its initial INVITE, so that the candidates
can be provided in the INVITE. This can be started upon
user interface cues that a call is pending, such as activity on a keypad or
the phone going off-hook.
On the receipt of the offer, the answerer SHOULD generate an answer in a
provisional response as soon as it has completed gathering
the candidates. ICE requires that a provisional response with an SDP be
transmitted reliably. This can be done through the existing
Provisional Response Acknowledgment (PRACK)
mechanism or through an ICE specific optimization, wherein,
the agent retransmits the provisional response with the exponential backoff
timers described in . Such retransmissions MUST cease on receipt
of a STUN Binding request with transport address matching candidate address
for one of the data streams signaled in that SDP or on transmission of the answer
in a 2xx response. If no Binding request is received prior to the last retransmit,
the agent does not consider the session terminated.
For the ICE lite peers , the agent MUST cease retransmitting the 18x after
sending it four times since there will be no Binding request sent and
the number four is arbitrarily chosen to limit the number of 18x retransmits
('poor man’s version of ' basically). (ICE will actually work even
if the peer never receives the 18x; however, experience has shown that sending
it is important for middleboxes and firewall traversal).
Once the answer has been sent, the agent SHOULD begin its connectivity checks.
Once candidate pairs for each component of a data stream enter the valid list,
the answerer can begin sending media on that data stream.
However, prior to this point, any media that needs to be sent towards the
caller (such as SIP early media ) MUST NOT be transmitted. For this
reason, implementations SHOULD delay alerting the called party until candidates
for each component of each data stream have entered the valid list.
In the case of a PSTN gateway, this would mean that the setup message into the
PSTN is delayed until this point. Doing this increases the post-dial delay, but
has the effect of eliminating 'ghost rings'.
Ghost rings are cases where the called party hears the phone ring, picks up, but
hears nothing and cannot be heard. This technique works without requiring support
for, or usage of, preconditions . It also has the benefit of guaranteeing
that not a single packet of media will get clipped, so that post-pickup delay is zero.
If an agent chooses to delay local alerting in this way, it SHOULD generate a 180
response once alerting begins.
In addition to uses where the offer is in an INVITE, and the answer is in the
provisional and/or 200 OK response, ICE works with cases where the offer appears
in the response.
In such cases, which are common in third party call control , ICE
agents SHOULD generate their offers in a reliable provisional response
(which MUST utilize ), and not alert the user on receipt of the INVITE.
The answer will arrive in a PRACK.
This allows for ICE processing to take place prior to alerting, so that there is no
post-pickup delay, at the expense of increased call setup delays.
Once ICE completes, the callee can alert the user and then generate a 200 OK
when they answer.
The 200 OK would contain no SDP, since the offer/answer exchange has completed.
Alternatively, agents MAY place the offer in a 2xx instead (in which case the
answer comes in the ACK).
When this happens, the callee will alert the user on receipt of the INVITE,
and the ICE exchanges will take place only after the user answers.
This has the effect of reducing call setup delay, but can cause substantial
post-pickup delays and media clipping.
specifies a SIP option tag and media feature tag for usage with ICE.
ICE implementations using SIP SHOULD support this specification, which uses a
feature tag in registrations to facilitate interoperability through signaling
intermediaries.
ICE interacts very well with forking.
Indeed, ICE fixes some of the problems associated with forking.
Without ICE, when a call forks and the caller receives multiple incoming
data streams, it cannot determine which data stream corresponds to
which callee.
With ICE, this problem is resolved.
The connectivity checks which occur prior to transmission of media carry
username fragments, which in turn are correlated to a specific callee.
Subsequent media packets that arrive on the same candidate pair as the
connectivity check will be associated with that same callee.
Thus, the caller can perform this correlation as long as it has received an answer.
Quality of Service (QoS) preconditions, which are defined in
and , apply only to the transport addresses listed as the default
targets for media in an offer/answer.
If ICE changes the transport address where media is received, this change
is reflected in an updated offer that changes the default destination for
media to match ICE’s selection. As such, it appears like any other re-INVITE would,
and is fully treated in RFCs 3312 and 4032, which apply without regard to the fact
that the destination for media is changing due to ICE negotiations occurring
"in the background".
Indeed, an agent SHOULD NOT indicate that QoS preconditions have been met
until the checks have completed and selected the candidate pairs to be used for media.
ICE also has (purposeful) interactions with connectivity preconditions .
Those interactions are described there.
Note that the procedures described in describe their own type of "preconditions", albeit with less functionality than those provided by the explicit preconditions in .
ICE works with Flows I, III, and IV as described in .
Flow I works without the controller supporting or being aware of ICE.
Flow IV will work as long as the controller passes along the ICE attributes without alteration.
Flow II is fundamentally incompatible with ICE; each agent will believe itself to be the answerer and thus never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of , require additional behavior of ICE implementations to support.
In particular, if an agent receives a mid-dialog re-INVITE that contains no offer, it MUST restart ICE for each data stream and go through the process of gathering new candidates.
Furthermore, that list of candidates SHOULD include the ones currently being used for media.
, the Alternative Network Address Types (ANAT) Semantics for the SDP grouping framework, and , its usage with SIP, define a mechanism for indicating that an agent can support both IPv4 and IPv6 for a data stream, and it does so by including two "m=" lines, one for v4 and one for v6. This is similar to ICE, which allows for an agent to indicate multiple transport addresses using the candidate attribute. However, ANAT relies on static selection to pick between choices, rather than a dynamic connectivity check used by ICE.
It is RECOMMENDED that ICE be used in realizing the dual-stack use-cases in agents that support ICE.
An attacker that can modify or disrupt the offer/answer exchanges themselves can readily launch a variety of attacks with ICE.
They could direct media to a target of a DoS attack, they could insert themselves into the data stream, and so on.
These are similar to the general security considerations for offer/answer exchanges, and the security considerations in apply.
These require techniques for message integrity and encryption for offers and answers, which are satisfied by the TLS mechanism when SIP is used.
As such, the usage of TLS with ICE is RECOMMENDED.
In addition to attacks where the attacker is a third party trying to insert fake offers, answers, or STUN messages, there are several attacks possible with ICE when the attacker is an authenticated and valid participant in the ICE exchange.
The voice hammer attack is an amplification attack.
In this attack, the attacker initiates sessions to other agents, and maliciously includes the IP address and port of a DoS target as the destination for media traffic signaled in the SDP.
This causes substantial amplification; a single offer/answer exchange can create a continuing flood of media packets, possibly at high rates (consider video sources).
This attack is not specific to ICE, but ICE can help provide remediation.
Specifically, if ICE is used, the agent receiving the malicious SDP will first perform connectivity checks to the target of media before sending media there.
If this target is a third-party host, the checks will not succeed, and media is never sent.
Unfortunately, ICE doesn’t help if it’s not used, in which case an attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients is known, and is limited to ones that support ICE, the server can reject any offers or answers that don’t indicate ICE support.
SIP User Agents (UA) that are not willing to receive non-ICE answers MUST include an "ice" Option Tag in the SIP Require Header Field in their offer.
UAs that rejects non-ICE offers SHOULD use a 421 response code, together with an Option Tag "ice" in the Require Header Field in the response.
Application Layer Gateways (ALGs) are functions present in a Network Address Translation (NAT) device that inspect the contents of packets and modify them, in order to facilitate NAT traversal for application protocols.
Session Border Controllers (SBCs) are close cousins of ALGs, but are less transparent since they actually exist as application-layer SIP intermediaries.
ICE has interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as long as the ALG correctly modifies the SDP.
A correct ALG implementation behaves as follows:
The ALG does not modify the "m=" and "c=" lines or the rtcp attribute if they contain external addresses.
If the "m=" and "c=" lines contain internal addresses, the modification depends on the state of the ALG:
If the ALG already has a binding established that maps an external port to an internal IP address and port matching the values in the "m=" and "c=" lines or rtcp attribute, the ALG uses that binding instead of creating a new one.
If the ALG does not already have a binding, it creates a new one and modifies the SDP, rewriting the "m=" and "c=" lines and rtcp attribute.
Unfortunately, many ALGs are known to work poorly in these corner cases.
ICE does not try to work around broken ALGs, as this is outside the scope of its functionality.
ICE can help diagnose these conditions, which often show up as a mismatch between the set of candidates and the "m=" and "c=" lines and rtcp attributes.
The ice-mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS.
This prevents the ALG from manipulating the SDP messages and interfering with ICE operation.
Implementations that are expected to be deployed behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the behavior of the SBC.
If it is acting as a proper Back-to-Back User Agent (B2BUA), the SBC will remove any SDP attributes it doesn’t understand, including the ICE attributes.
Consequently, the call will appear to both endpoints as if the other side doesn’t support ICE.
This will result in ICE being disabled, and media flowing through the SBC, if the SBC has requested it.
If, however, the SBC passes the ICE attributes without modification, yet modifies the default destination for media (contained in the "m=" and "c=" lines and rtcp attribute), this will be detected as an ICE mismatch, and ICE processing is aborted for the call.
It is outside of the scope of ICE for it to act as a tool for "working around" SBCs.
If one is present, ICE will not be used and the SBC techniques take precedence.
The original ICE specification defined seven new SDP attributes per the procedures of
Section 8.2.4 of . The registration information from the original specification
is included here with modifications to include Mux Category and also defines
a new SDP attribute 'ice-pacing'.
candidate
media-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and provides one of many possible candidate addresses for communication.
These addresses are validated with an end-to-end connectivity check using Session
Traversal Utilities for NAT (STUN).
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
TRANSPORT
remote-candidates
media-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and provides the identity of the remote candidates that the offerer wishes the answerer
to use in its answer.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
TRANSPORT
ice-lite
session-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and indicates that an agent has the minimum functionality required to support ICE
inter-operation with a peer that has a full implementation.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
NORMAL
ice-mismatch
media-level
No
This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent is ICE capable, but did not proceed with ICE due to a mismatch of candidates with the default destination for media signaled in the SDP.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
NORMAL
ice-pwd
session- or media-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and provides the password used to protect STUN connectivity checks.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
TRANSPORT
ice-ufrag
session- or media-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and provides the fragments used to construct the username in STUN connectivity checks.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
TRANSPORT
ice-options
ice-options
session-level
No
This attribute is used with Interactive Connectivity Establishment (ICE),
and indicates the ICE options or extensions used by the agent.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
NORMAL
This specification also defines a new SDP attribute, "ice-pacing" according
to the following data:
ice-pacing
session-level
No
This attribute is used with Interactive Connectivity Establishment (ICE)
to indicate desired connectivity check pacing values.
See of RFC XXXX.
IESG
iesg@ietf.org
RFCXXXX
NORMAL
IANA maintains a registry for ice-options identifiers under the Specification
Required policy as defined in "Guidelines for Writing an IANA Considerations
Section in RFCs" .
ICE options are of unlimited length according to the syntax in
; however, they are RECOMMENDED to be no longer
than 20 characters. This is to reduce message sizes and allow for
efficient parsing. ICE options are defined at the session leve..
A registration request MUST include the following information:
The ICE option identifier to be registered
Name, Email, and Address of a contact person for the registration
Organization or individuals having the change control
Short description of the ICE extension to which the option relates
Reference(s) to the specification defining the ICE option and the related extensions
A large part of the text in this document was taken from , authored by
Jonathan Rosenberg.
Some of the text in this document was taken from , authored by Magnus Westerlund
and Colin Perkins.
Many thanks to Christer Holmberg for providing text suggestions in Section 3 that aligns
with
Thanks to Thomas Stach for text help, Roman Shpount for suggesting RTCP candidate
handling and Simon Perreault for advising on IPV6 address selection when candidate-address
includes FQDN.
Many thanks to Flemming Andreasen for shepherd review feedback.
Thanks to following experts for their reviews and constructive feedback: Christer Holmberg,
Adam Roach, Peter Saint-Andre and the MMUSIC WG.
Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) TraversalThis document describes a protocol for Network Address Translator (NAT) traversal for UDP-based communication. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).This document obsoletes RFC 5245.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.SIP: Session Initiation ProtocolThis document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK]Reliability of Provisional Responses in Session Initiation Protocol (SIP)This document specifies an extension to the Session Initiation Protocol (SIP) providing reliable provisional response messages. This extension uses the option tag 100rel and defines the Provisional Response ACKnowledgement (PRACK) method. [STANDARDS-TRACK]An Offer/Answer Model with Session Description Protocol (SDP)This document defines a mechanism by which two entities can make use of the Session Description Protocol (SDP) to arrive at a common view of a multimedia session between them. In the model, one participant offers the other a description of the desired session from their perspective, and the other participant answers with the desired session from their perspective. This offer/answer model is most useful in unicast sessions where information from both participants is needed for the complete view of the session. The offer/answer model is used by protocols like the Session Initiation Protocol (SIP). [STANDARDS-TRACK]Integration of Resource Management and Session Initiation Protocol (SIP)Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) BandwidthThis document defines an extension to the Session Description Protocol (SDP) to specify two additional modifiers for the bandwidth attribute. These modifiers may be used to specify the bandwidth allowed for RTP Control Protocol (RTCP) packets in a Real-time Transport Protocol (RTP) session. [STANDARDS-TRACK]Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)The Session Description Protocol (SDP) is used to describe the parameters of media streams used in multimedia sessions. When a session requires multiple ports, SDP assumes that these ports have consecutive numbers. However, when the session crosses a network address translation device that also uses port mapping, the ordering of ports can be destroyed by the translation. To handle this, we propose an extension attribute to SDP.Update to the Session Initiation Protocol (SIP) Preconditions FrameworkThis document updates RFC 3312, which defines the framework for preconditions in SIP. We provide guidelines for authors of new precondition types and describe how to use SIP preconditions in situations that involve session mobility. [STANDARDS-TRACK]The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping FrameworkThis document defines the Alternative Network Address Types (ANAT) semantics for the Session Description Protocol (SDP) grouping framework. The ANAT semantics allow alternative types of network addresses to establish a particular media stream. [STANDARDS-TRACK]Usage of the Session Description Protocol (SDP) Alternative Network Address Types (ANAT) Semantics in the Session Initiation Protocol (SIP)This document describes how to use the Alternative Network Address Types (ANAT) semantics of the Session Description Protocol (SDP) grouping framework in SIP. In particular, we define the sdp-anat SIP option-tag. This SIP option-tag ensures that SDP session descriptions that use ANAT are only handled by SIP entities with ANAT support. To justify the need for such a SIP option-tag, we describe what could possibly happen if an ANAT-unaware SIP entity tried to handle media lines grouped with ANAT.[STANDARDS-TRACK]SDP: Session Description ProtocolThis memo defines the Session Description Protocol (SDP). SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. [STANDARDS-TRACK]Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication transform for IPsec). To ensure that such quantities have consistent values and interpretations across all implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA).In order for IANA to manage a given namespace prudently, it needs guidelines describing the conditions under which new values can be assigned or when modifications to existing values can be made. If IANA is expected to play a role in the management of a namespace, IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a namespace and provides guidelines for authors on the specific text that must be included in documents that place demands on IANA.This document obsoletes RFC 2434. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)This specification defines a media feature tag and an option tag for use with the Session Initiation Protocol (SIP). The media feature tag allows a User Agent (UA) to communicate to its registrar that it supports ICE. The option tag allows a UA to require support for ICE in order for a call to proceed. [STANDARDS-TRACK]IANA Registry for Interactive Connectivity Establishment (ICE) OptionsIt has been identified that "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols" (RFC 5245) is missing a registry for ICE
options. This document defines this missing IANA registry and
updates RFC 5245.Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)Third party call control refers to the ability of one entity to create a call in which communication is actually between other parties. Third party call control is possible using the mechanisms specified within the Session Initiation Protocol (SIP). However, there are several possible approaches, each with different benefits and drawbacks. This document discusses best current practices for the usage of SIP for third party call control. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)This document describes how to manage early media in the Session Initiation Protocol (SIP) using two models: the gateway model and the application server model. It also describes the inputs one needs to consider in defining local policies for ringing tone generation. This memo provides information for the Internet community.Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer ProtocolsThis document describes a protocol for Network Address Translator (NAT) traversal for UDP-based multimedia sessions established with the offer/answer model. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN). ICE can be used by any protocol utilizing the offer/answer model, such as the Session Initiation Protocol (SIP). [STANDARDS-TRACK]Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)The Session Initiation Protocol (SIP) allows proxy servers to initiate TCP connections or to send asynchronous UDP datagrams to User Agents in order to deliver requests. However, in a large number of real deployments, many practical considerations, such as the existence of firewalls and Network Address Translators (NATs) or the use of TLS with server-provided certificates, prevent servers from connecting to User Agents in this way. This specification defines behaviors for User Agents, registrars, and proxy servers that allow requests to be delivered on existing connections established by the User Agent. It also defines keep-alive behaviors needed to keep NAT bindings open and specifies the usage of multiple connections from the User Agent to its registrar. [STANDARDS-TRACK]Connectivity Preconditions for Session Description Protocol (SDP) Media StreamsThis document defines a new connectivity precondition for the Session Description Protocol (SDP) precondition framework. A connectivity precondition can be used to delay session establishment or modification until media stream connectivity has been successfully verified. The method of verification may vary depending on the type of transport used for the media. For unreliable datagram transports such as UDP, verification involves probing the stream with data or control packets. For reliable connection-oriented transports such as TCP, verification can be achieved simply by successful connection establishment or by probing the connection with data or control packets, depending on the situation. [STANDARDS-TRACK]
For the example shown in section 15 of the resulting offer (message 5) encoded in SDP looks like:
The offer, with the variables replaced with their values, will look like (lines folded for clarity):
The resulting answer looks like:
With the variables filled in:
The a=remote-candidates attribute exists to eliminate a race condition between the updated offer and the response to the STUN Binding request that moved a candidate into the Valid list.
This race condition is shown in .
On receipt of message 4, agent L adds a candidate pair to the valid list.
If there was only a single data stream with a single component, agent L could now send an updated offer.
However, the check from agent R has not yet generated a response, and agent R receives the updated offer (message 7) before getting the response (message 9).
Thus, it does not yet know that this particular pair is valid.
To eliminate this condition, the actual candidates at R that were selected by the offerer (the remote candidates) are included in the offer itself, and the answerer delays its answer until those pairs validate.
|
|(2) Answer | |
||
|(4) STUN Res. | |
|| |
| |Lost |
|(7) Offer | |
|------------------------------------------>|
|(8) STUN Req. | |
||
|(10) Answer | |
|
When ICE runs between two peers, one agent acts as controlled, and the other as controlling.
Rules are defined as a function of implementation type and offerer/answerer to determine who is controlling and who is controlled.
However, the specification mentions that, in some cases, both sides might believe they are controlling, or both sides might believe they are controlled.
How can this happen?
The condition when both agents believe they are controlled shows up in third party call control cases.
Consider the following flow:
| |
| |(3) INV() |
| |------------->|
| |(4) 200(SDP2) |
| ||
]]>
This flow is a variation on flow III of RFC 3725 .
In fact, it works better than flow III since it produces fewer messages.
In this flow, the controller sends an offerless INVITE to agent A, which responds with its offer, SDP1.
The agent then sends an offerless INVITE to agent B, which it responds to with its offer, SDP2.
The controller then uses the offer from each agent to generate the answers.
When this flow is used, ICE will run between agents A and B, but both will believe they are in the controlling role.
With the role conflict resolution procedures, this flow will function properly when ICE is used.
At this time, there are no documented flows that can result in the case where both agents believe they are controlled.
However, the conflict resolution procedures allow for this case, should a flow arise that would fit into this category.
Section 11.1 describes rules for sending media.
Both agents can send media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP so that the default destination for media matches where media is being sent based on ICE procedures (which will be the highest-priority nominated candidate pair).
This begs the question — why is the updated offer/answer exchange needed at all?
Indeed, in a pure offer/answer environment, it would not be.
The offerer and answerer will agree on the candidates to use through ICE, and then can begin using them.
As far as the agents themselves are concerned, the updated offer/answer provides no new information.
However, in practice, numerous components along the signaling path look at the SDP information.
These include entities performing off-path QoS reservations, NAT traversal components such as ALGs and Session Border Controllers (SBCs), and diagnostic tools that passively monitor the network.
For these tools to continue to function without change, the core property of SDP — that the existing, pre-ICE definitions of the addresses used for media — the "m=" and "c=" lines and the rtcp attribute — must be retained.
For this reason, an updated offer must be sent.
Following experts have contributed textual and structural improvements for this work
Christer Holmberg
Ericsson
Email: christer.holmberg@ericsson.com
Roman Shpount
TurboBridge
rshpount@turbobridge.com
Thomas Stach
thomass.stach@gmail.com